(1) Field of the Invention
The invention relates to ion exchange resins and their use in the removal of chromium from water.
(2) Description of the Related Art
Chromium is present in many water sources including ground and surface water. Hexavalent chromium, in its various forms (e.g. HCrO4−, CrO4−2, and Cr2O7−2), is soluble over a wide range of pH values. As a consequence, the number of effective chromium removal techniques is limited. One conventional method is treatment with ion exchange resin such as DOWEX™ 1 brand ion exchange resin, a strong base, gel-type resin comprising a crosslinked styrene-divinylbenzene copolymer matrix with quaternary ammonium functionality (i.e. functional groups produced via reaction between a chloromethylated styrene-divinylbenzene copolymer matrix and a trimethyl amine). Other examples of ion exchange resins used in chromium removal include: DOWEX™ MAC-3 brand ion exchange resin, a macroporous resin including an acrylic matrix with carboxylic functional groups; DOWEX™ SAR brand ion exchange resin, a gel-type resin including a styrene-divinylbenzene matrix with quaternary ammonium functionality (i.e. functional groups produced via reaction between a chloromethylated styrene-divinylbenzene copolymer matrix and a dimethylethanol amine); and DOWEX™ M4195 brand ion exchange resin, a macroporous chelating resin including a styrene-divinylbenzene matrix with chelation groups produced via reaction with bis-picolylamine—all commercially available from The Dow Chemical Company. Other types of ion exchange resin are also known for use in chromium removal, e.g. phenol-formaldehyde functionalized with secondary amines, and epoxy polyamines.
In the presence of competing ions (e.g. chloride, sulfate, bicarbonate, etc.), the chromium removal capacity of an ion exchange resin is typically improved at acidic pH values, e.g. typically below a pH of 6.3. As a consequence, conventional treatment regimes involve reducing the pH of the water source prior to treatment with ion exchange resin. See for example: (1) Ion Exchange Technology—Advances in Pollution Control, ed. A. Sengupta, Technomic Publishing Co. (1995), Lancaster Pa. (ISBN No. 1-56676-241-3), see specifically Chapter 3, “Chromate Ion Exchange”; and (2) Ion Exchange Developments and Applications, Proceedings of IEX '96, ed. J. A. Greig. The Royal Society of Chemistry (1996), Cambridge UK (ISBN 0-85404-726-3), see specifically pg. 388-395, “Kinetics of Chromic Acid Removal by Anion Exchange” by H. K. S. Tan. The step of reducing the pH of a water source prior to ion exchange treatment is both time consuming and costly.